Exchange token tests

[srushtisv]

Description

This PR introduces unit tests for the ExchangeTokenRequest.swift file, which was added to support the Bring Your Own CIAM (BYO-CIAM) feature. These tests verify that the request to the regionalized identityplatform.googleapis.com service for exchanging OIDC tokens is constructed correctly.

Tests Added:

• Verification of the full URL for different regions, including the special prod-global case, for both production and staging (useStaging) environments. This includes the host, the /v2beta/ API version path, project, location, tenant, and IDP config segments.
• Validation of the unencodedHTTPRequestBody to ensure it contains the id_token.

To ensure the reliability and correctness of the network requests made during the OIDC token exchange flow, which is critical for the BYO-CIAM functionality.
This PR only adds test code and does not affect the SDK's public API or runtime behavior.

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